How can I calculate the force of a bullet shot against a wall if the velocity v and mass m are given?

The mass and velocity only give you kinetic energy: E=m*v^2/2
The force is not yet determined, since it depends on the length of the path of deceleration. Let's say that a bullet does not deform itself and penetrates a distance of s into the wall. Then the force (averaged over the deceleration path) is F=E/s=m*v^2/(2s). A hard wall will stop the bullet faster (smaller s) and produce greater force. (however the force comes before acceleration, so it would be more consistent to say that a hard wall will act with a greater force and so stop the bullet faster).
If the wall is very hard (bullet deformation is comparable or larger than wall deformation), then one should take the wall penetration depth+bullet deformation for s (although the exact calculation would be more difficult, since this is no longer a rigid body movement).